Determinants of blood pressure and cardiac output (preload, afterload, contractility, heart rate)
Core concept
- MAP = CO x SVR and CO = HR x SV
- MAP ~ DBP + 1/3 pulse pressure
- SV determined by preload, afterload and contractility - nothing else
- Preload = end-diastolic sarcomere stretch (approximated by LVEDV, then LVEDP, then PAWP, then CVP - each step loses accuracy)
- Frank-Starling: inc stretch -> inc troponin C calcium sensitivity and optimal actin-myosin overlap -> inc force
- Determined by venous return - blood volume, venous tone, intrathoracic pressure, atrial contribution (~20-30%, up to 40% in a stiff ventricle), heart rate/filling time, pericardial constraint
- Afterload = wall stress during ejection
- Laplace: wall stress = (P x r) / 2h -> dilatation raises afterload, hypertrophy reduces it
- Determined by SVR, arterial compliance, aortic impedance, LV size, and outflow obstruction (AS, HOCM)
- Contractility (inotropy) = force at any given preload and afterload
- inc by sympathetic beta-1/cAMP, catecholamines, digoxin, calcium
- dec by ischaemia, acidosis, hypoxia, beta-blockade, sepsis, hypocalcaemia
3 more sections, plus exam facts
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